1. Field of the Invention
The present invention relates to a distance measuring system which measures the distance between two different points based on an arrival time difference of pulse signals.
2. Description of the Related Art
There has been proposed a distance measuring system which uses radio waves as a so-called ranging method of measuring the distance between two different points.
In the distance measuring system, as shown in FIG. 8, for example, when measuring a distance 1 from a point F to a point G is desired, a transmitting device 61 and a receiving device 62 are respectively placed at the point F and the point G as radio stations. Then, the transmitting device 61 transmits a radio wave comprised of pulse signals, and the receiving device 62 receives the radio wave. At this time, an arrival time Td for the pulse signals transmitted from the transmitting device 61 to be received by the receiving device 62 is measured. The distance 1 can be calculated by multiplying the measured arrival time Td by a propagation velocity Vc of the radio wave. Because the propagation velocity Vc of the radio wave is constant, the distance 1 can be measured accurately by measuring only the arrival time Td.
As a method of measuring the propagation time Td, there has been proposed a method which uses spectral spread technique of measuring a distance between radio devices at a phase timing of a spread code in transmission and reception in addition to a method of measuring a time at which the pulse signals zero-cross, a method of measuring the arrival time Td by identifying a phase difference between the pulse signals.
FIG. 9 shows the configuration of a conventional system using the spectral spread technique. The system measures a distance d between a radio device 71 and a radio device 72 which are located at two different points, and is configured so that the radio device 71 includes a transmission section 82 which transmits a signal comprised of a pulse sequence or the like, an antenna 83 connected to the transmission section 82, an antenna 84 for receiving a radio wave from the radio device 72, a reception section 85 connected to the antenna 84, a correlation calculating section 86 connected to the reception section 85, a correlation position determining section 87 connected to the correlation calculating section 86, and finally a distance measuring section 88 connected to the transmission section 82 and the correlation position determining section 87 to measure the distance d. A local transmission signal is supplied to the transmission section 82 and the reception section 85 from the local oscillator 81.
The radio device 72 includes an antenna 91 for receiving a radio wave transmitted from the antenna 83, a reception section 92 connected to the antenna 91, a transmission section 94 which transmits data, and an antenna 95 connected to the transmission section 94 to transmit a radio wave. A local transmission signal is supplied to the transmission section 92 and the reception section 94 from a local oscillator 93.
A signal represented by, for example, a spread code of a base band and a phase timing, is generated by the signal generating section 81 in the radio device 71, and is converted to a high-frequency signal with a center frequency f1 by the transmission section 82, and the signal is sent to the radio device 72 via the antenna 83.
In the radio device 72, a high-frequency spread code received via the antenna 91 is amplified by the reception section 92, and is converted to have a center frequency f2 by an unillustrated frequency converting section to generate a signal to be transmitted to the radio device 71, and this signal is returned to the radio device 71 via the transmission section 94. The radio device 71 receives the spread code resent via the antenna 84 and the reception section 85, and converts the high-frequency spread code to have a base band by orthogonal detection. Further, the correlation calculating section 86 performs an autocorrelation operation on the spread code, and the correlation position determining section 87 detects the phase timing of the received spread code based on an autocorrelation peak position. The distance measuring section 88 detects a difference T1 between the phase timing of the transmitted spread code and the phase timing of the received spread code to calculate the distance d between the radio devices 71 and 72. That is, the system configuration shown in FIG. 9 executes a so-called TWR (Two Way Ranging) type measurement of calculating the distance by causing signals to reciprocate between the radio devices 71 and 72, and dividing the acquired difference by two.
The transmission section 82, 94 which send a signal to be transmitted and received produces local oscillation signals by means of the local oscillator 81, 93 which comprises an oscillator to accurately keep time, and adds the local oscillation signals to produce a pulse sequence. It is therefore ideally desirable that the times kept by the local oscillator 81 and the local oscillator 93 are identical, but actually they are often deviate from each other by a unit of ppm (1/1,000,000).
FIG. 10 shows the relationship of measuring errors with respect to an offset A (ppm) of the local oscillator 81 and an offset B (ppm) of the local oscillator 93.
When there is a relative time difference of approximately 10 ppm between the local oscillator 81 and the local oscillator 93, as shown in FIG. 10, the measured distance has an error of 3 m to the actual distance.
There has also been proposed a technique of eliminating such a measuring error. In this regard, please see 1) V. Brethour, “Two Way Ranging using Tracking Information to Manage Crystal Offset,” IEEE 802.15 WPAN documents, 15-05-0336-r00; 2) R. Hach, “Symmetric Double Side-Two Way Ranging,” IEEE 802.15 WPAN documents, 15-05-0334-r00; and 3) R. Roberts, “Ranging Subcommittee Final Report,” IEEE 802.15 WPAN documents, 15-04-0581-r07.